The present invention relates to a method of manufacturing a web of paper with a headbox.
A headbox establishes the density and transverse fiber orientation of a pulp suspension at a point at least just upstream of the outlet gap from the headbox to attempt to achieve a paper weight per area and transverse fiber orientation which are satisfactory and constant over the entire width of the web. A headbox simultaneously also ensures that the constitution of the web is homogeneous. The constitution of the paper web is comprised of concentrations of its individual constituents, such as long and short fibers, fillers, etc.
The composition of a web of paper transversely to its length direction, i.e. across a web being produced, can be influenced by the process known from German Patent 3,514,554, for example. This patent describes varying the density of the pulp at particular points when necessary. But, no description is provided about how this is to be done.
German Patent 4,019,593 A1 describes varying the concentration C.sub.M of the particular sectional stream and accordingly of the material flowing out of the particular mixer in the event of deviations in weight at a particular point along the width of the web. To attain this, the ratio Q.sub.H /Q.sub.L of the volumes of control streams supplied to the mixer is varied. It is difficult, however, with conventional valves to prevent the sectional stream Q.sub.H leaving the mixer from deviating unintentionally and wildly from an ideal concentration.
German Application OS 3,538,466 describes varying the flow through one section of a headbox to control the angle of fiber orientation where the pulp leaves the headbox. Fiber orientation will also vary wildly when the volume of a sectional stream deviates wildly from the ideal orientation.
German Patent 2,942,966 and OS 3,538,466 disclose varying the width of the outlet gap or slice or slot of the headbox with threaded spindles that pivot or curve the upper blade of the outlet gap, for example. This approach can vary the local flow of suspension and hence the fiber orientation. Local narrowing of the gap changes the direction in which the fibers are flowing at the constricted points as compared with other points across the gap. This means that, although the weight of the web can be made uniform over the width of the headbox outlet by this "displacement", the generally necessary overall orientation of the fibers in the web will be disrupted.